1. Field of the Invention
This invention relates to invasive medical apparatus, and more specifically to surgical instruments which can be used safely during magnetic resonance imaging.
2. Description of Related Art
A biopsy needle is an invasive device used in surgical procedures to penetrate a subject in order to reach a desired tissue and extract a small portion of the tissue for analysis. Another invasive device, a laser fiber guide, is a hollow tube also used in surgical procedures to penetrate a subject to a desired location, and when at that location, an optical fiber can be inserted through the laser fiber guide with one end positioned at the desired location. Optical radiation, such as laser radiation, is introduced at another end of the optical fiber so as to pass through the fiber and impinge on tissue at the desired location for the purpose of cauterizing or destroying the tissue at that location. These biopsy needles and fiber guides must be thin enough to penetrate the subject, stiff enough not to bend, made of a material which can be sterilized and does not cause a significant adverse physiological reaction when introduced into a living subject. Conventional invasive devices are constructed of a material, usually metal, such as stainless steel. Several other alloys are well suited for the construction of such invasive devices.
Magnetic resonant (MR) imaging employs large magnets for creating a homogeneous magnetic field, and gradient coils for altering the magnetic field in a uniform manner in time or space, creating magnetic field gradients. MR imaging also employs radiofrequency (RF) coils for applying an RF field to tissue to be imaged, causing the tissue to resonate and create an MR response signal. The MR response signal is used to construct an image. The degree of homogeneity of the magnetic field and the linearity of a magnetic field gradient over space are important in creating a clear, undistorted image. Interference with the RF field also reduces the quality of the created image.
Recently, there is a desire to create interactive images of internal organs of a patient during surgery. Since magnetic resonance imaging provides great detail in images of soft tissues, it is advantageous to use MR imaging. The best imaging results when surgical equipment does not interfere with the magnetic and RF fields created by the MR imaging equipment.
Many metals are ferromagnetic and are physically pulled toward a magnet. Since the magnetic field employed in MR imaging is large, an amount of magnetic force applied to the equipment can be large. Invasive devices, therefore, should not be made of a ferromagnetic material since a magnetic force would be applied to them causing them to be difficult to manipulate.
Other problems occur with materials in which eddy currents are produced when placed in a variable magnetic field. The eddy currents in these materials, usually conductors, create their own magnetic field which interferes with the RF field used for MR imaging. Therefore, materials which exhibit eddy currents, such as aluminium and copper, should not be used within a variable magnetic field.
The degree of magnetization the material exhibits per applied magnetic field is defined as susceptibility. The susceptibility of a material also affects the homogeneity of the applied magnetic field in a region surrounding the material. This can create large distortions in an MR image near the material.
Many materials used for invasive devices which do not exhibit significant susceptibility tend to be brittle and would be likely to fracture when inserted into a subject. This would cause pieces of the device to remain in the subject and not be easily removed. Surgery would likely be the only method of removing such pieces.
Currently, there is a need for surgical instruments which can be used within a MR magnetic field and not be physically pulled toward the magnet creating the MR field, not interfere with a created MR image, and not present a danger of breakage when inserted into a subject.